Lean-burn engines are supplied with, and combust, a lean mixture of air and fuel (oxygen-rich mixture) to achieve more efficient fuel economy. Some notable examples of lean-burn engines include charge compression-ignition (diesel) engines, certain spark-ignition (gasoline) engines such as spark-ignition direct injection (SIDI) engines, and homogeneous charge compression ignition (HCCI) engines. These and other types of lean-burn engines are generally more fuel efficient than conventional rich-burn spark-ignition engines as a result of thermal efficiency gains related to, among other factors, decreases in throttle and heat losses.
The exhaust emitted from a lean-burn engine during periods of lean-burn operation may include a relatively high content of nitrogen (N2) and oxygen (O2), a relatively low content of carbon monoxide (CO) and unburned/partially-burned hydrocarbons (HC's), possibly some suspended particulate matter (i.e., in diesel engines), and small amounts of nitrogen oxides primarily comprised of NO and NO2 (collectively referred to as NOX). The NOX constituency of the exhaust may fluctuate between about 50 and about 1500 ppm and may comprise greater than 90 mol % NO and less than 10 mol % NO2 along with nominal amounts of N2O. The hot engine exhaust, which can reach temperatures of up to about 900° C., often needs to be treated to decrease the concentration of CO, HC's, NOX, and particulate suspensions before it can be released to the atmosphere.
An exhaust aftertreatment system may be configured downstream of a lean-burn engine to manage any unwanted gaseous emissions and possible particulate matter contained in the engine's exhaust. A typical exhaust aftertreatment system usually aspires to cooperatively (1) oxidize CO into carbon dioxide (CO2), (2) oxidize HC's into CO2 and water (H2O), (3) convert NOX gases into nitrogen (N2) and O2, and (4) filter off or otherwise destroy any suspended particulate matter. A variety of exhaust aftertreatment system architectures that employ specially-catalyzed components have been devised and are able to sufficiently facilitate these reactions so that the exhaust expelled to the environment contains a much more desirable chemical makeup. The various components that make up the exhaust aftertreatment system may vary depending on, among other factors, the type of lean-burn engine being operated.
A SIDI engine, for example, may be connected to a catalytic converter that houses a three-way-catalyst comprised of platinum group metals (PGM's) such as platinum, palladium, and rhodium. Catalytic converters have long been used in conventional spark-ignition gasoline engines that combust a near-stoichiometric mixture of air and fuel to remove unwanted CO, HC's, and NOX from the engine's exhaust. But the nature of the exhaust produced from a SIDI engine during periods of lean-burn operation poses certain challenges with regard to NOX removal. One specific challenge is that the relatively high content of O2 and the relatively low content of CO and HC's in the hot exhaust renders the reaction kinetics for the conversion of NOX to N2 over PGM's quite unfavorable. As another example, a diesel engine may be connected to a diesel particulate filter and a diesel oxidation convertor that houses a diesel oxidation catalyst comprised of PGM's such as platinum and palladium. The diesel particulate filter and the diesel oxidation convertor can remove unwanted CO, HC's, and suspended particulates but, much like catalytic converters, are generally not suitable for removing NOX due to relatively high O2 concentrations. Additional measures are therefore generally incorporated into the exhaust aftertreatment systems of both SIDI and diesel engines, and all other types of lean-burn engines, to help remove NOX.
One available option that may be employed in an exhaust aftertreatment system of a lean-burn engine to help convert NOX to N2 is a selective catalyst reduction (SCR) catalyst. The SCR catalyst may be washcoated onto a support substrate located in the flow path of the exhaust. A reductant, such as a hydrocarbon or ammonia (NH3), may be introduced into and mixed with the exhaust expelled from the lean-burn engine upstream of the SCR catalyst. The reductant, once exposed to the SCR catalyst, selectively reduces NO to N2 in the presence of O2. The SCR catalyst may be positioned in the exhaust aftertreatment system downstream of an oxidation catalyst that oxidizes NO to NO2 in more than nominal quantities. This positioning of the SCR catalyst may be quite helpful since the SCR catalyst generally converts NOX to N2 more efficiently, especially at lower temperatures, as the molar ratio of NO to NO2 decreases from that which is originally generated by the lean-burn engine to a ratio of about 1.
Conventional diesel oxidation catalysts and two-way-catalysts are generally able to sufficiently affect the molar ratio of NO to NO2 in the NOX fed to the SCR converter. Conventional three-way-catalysts, however, generally possess a limited NO oxidation capability due to a lesser proportional platinum loading. As a result, to help maximize NOX conversion, an exhaust aftertreatment system for a diesel engine may simply position the SCR converter downstream of the diesel oxidation converter and an exhaust aftertreatment system for a SIDI engine may further incorporate a diesel oxidation catalyst or some other suitable two-way-catalyst upstream of the SCR converter. But the PGM's commonly used to make the oxidation catalyst for NO oxidation, most notably platinum, are quite expensive and have been shown, in some instances, to exhibit poor thermal durability when exposed to relatively high-temperature engine exhaust.
The use of a SCR converter in the exhaust aftertreatment system for a lean-burn engine is thus an attractive, yet challenging, option for removing NOX from the engine's exhaust. Such a device is often paired with an upstream oxidation catalyst to help boost NOX conversion at low-temperatures. Conventional oxidation catalysts, however, primarily contain expensive and insufficiently durable PGM's. SCR technology related to NOX removal is thus constantly in need of innovative developments and contributions that can help advance to this and other related fields of technological art.